In the field of construction machines, there have been heretofore proposed a variety of means for protecting the operator from overturn or tumbling of a construction machine by restricting or preventing deformation of the operator's cab. One example of such means is disclosed in Japanese Unexamined Published Patent Application No. 2000-230255 (First Example). This publication describes, in pages 2 to 3 and FIG. 1, a structure for a construction machine in which an operator's cab is mounted on either the right or left side of a revolving superstructure frame which is mounted on the upper part of the lower traveling structure so as to be freely turnable. This publication also describes a supporting structure in which a supporting member is attached to a side face of the revolving superstructure frame on the side of the operator's cab such that if the construction machine turns over on the operator's cab side, the supporting member holds the construction machine in cooperation with the lower traveling structure so as to keep the operator's cab out of touch with the ground. The first example provides a means that is inexpensive and easily applicable to ordinary construction machines, but suggests no idea for protection if the construction machine does not stay in an overturned state but goes into tumbling.
Another means is disclosed in Japanese Unexamined Published Patent Application No. 2001-173017 (Second Example). This publication describes, in pages 4 to 5 and FIGS. 1 and 4, a protective structure according to which the operator's cab is enclosed by a cab guard constructed by a portal-shaped frame and the operator's cab is protected by the cab guard in case of tumbling of the construction machine. With reference to FIGS. 13 and 14, there will be explained one example of construction machines having a cab protecting structure according to the prior art. When the terms “right” and “left”, are used herein, it should be understood that these terms have reference to the structure of the construction machine as it would appear to the operator looking forward from the vehicle (see FIG. 1).
Referring to FIGS. 13 and 14, a hydraulic excavator 80 is constructed such that a revolving superstructure frame 82 is mounted on a lower traveling structure 81 so as to be freely turnable. Mounted on the revolving superstructure frame 82 are a boom 83 for a work implement that is attached to the cross-wise mid position of the front part of the frame 82; a counterweight 84 that is located in the rear part of the frame 82; and an operator's cab 85 that is located in the left front part of the frame 82. A cab guard 90 encloses the operator's cab 85.
In the structure of the hydraulic excavator 80, the provision of the cab guard 90 enclosing the operator's cab 85 has revealed the following problem. The cab guard 90 is required to have an extremely strong structure in order to prevent or restrict deformation of the operator's cab 85 in the event of tumbling of the hydraulic excavator 80. For this reason, the cab guard 90 becomes extremely costly and an increase in the weight of the operator's cab leads to an increase in the total weight of the vehicle, resulting in a decrease in the operability of the vehicle. In addition, the visibility of the operator's cab 85 is interrupted by the cab guard 90, which leads to decreased work efficiency.